1. Field of the Invention
The present invention relates to an organic thin film transistor and in particular to a method of controlling the threshold voltage of an organic thin film transistor.
2. Related Art
Thin film transistors (TFTs) have been commercialized as switching elements in active matrix liquid crystal displays and the like, and are fabricated using amorphous or polycrystalline silicon as a semiconductor.
In recent years, much attention has been placed on organic semiconductor materials as the semiconductor material for TFTs. Organic semiconductors can be easily formed in a thin film using simple techniques such as spin coating and vacuum vapor deposition, and there is also the advantage that the manufacturing process can be carried out at a lower temperature than for conventional TFTs in which amorphous or polycrystalline silicon is used. If the process temperature is reduced, it becomes possible to form a TFT on a plastic substrate that normally has low heat resistance, which has great potential for reducing the weight and cost of displays and also for greater variety in the applications of TFTs due to the flexibility of plastic substrates.
However, when TFTs have hitherto been developed using organic semiconductor materials, it has been difficult to control the threshold voltage by doping with impurities in the same way as when manufacturing a TFT that uses amorphous or polycrystalline silicon, and this has been an obstacle for commercialization.
Threshold voltages are described by an article by Jiyoul Lee et al (see APPLIED PHYSICS LETTERS, Vol. 80, 2925-2927 (2002), for example), but a technique for freely controlling threshold voltages is not described.
It is an object of the present invention to provide an organic thin film transistor whose threshold voltage can be easily controlled, without changing the material forming an organic semiconductor film, by providing a threshold voltage controlling film between the gate insulating film and the organic semiconductor film, and a method of manufacturing the same.